Method and means for improving machinability of ferrous metals



Patented Mar. 11, 1941 UNITED STATES METHOD AND IVIEAN S FOR ILIPROVING MA- CHINABILITY F FERROUS METALS FrederickF. McIntosh, Sewickley, Pa., assignor to C b e S l C p y of America, New York,

N. Y., a corporation of New Jersey No Drawing.

Application May 13, 1939,

Serial N 0. 273,413

10 Claims.

This invention pertains to free machining, lead-containing ferrous metals and alloys, principally steel, and to improved methods and means for introducing the lead into and in finely dispersed form throughout the ferrous metal.

It is 'common practice to add elements, such as sulfur, selenium or tellurium, to iron or steel for increasing the machinability. Such additions are subject to the disadvantages, however, that they form non-metallic compounds distributed throughout the metal resulting in an unclean steel, and often a steel characterized by red shortness. To minimize these defects, other elements, such as manganese, molybdenum, chromium and zirconium, are sometimes added.

Increased machinability may also be secured by additions of lead to the steel while molten. Due to the fact, however, that lead and steel are mutually insoluble in one another in the solid state, although very slightly soluble while molten, no satisfactory method or means has heretofore'been devised, in so far as I am aware, for, getting the lead into the steel with the requisite uniformity of dispersion and in a sufin the past has been to introduce metallic lead in a fine state of subdivision into a stream of molten steel entering an ingot mold. In this way, turbulence of admixture is secured, a small amount of the lead passing into solution in the molten steel, a further portion being trapped in mechanical suspension therein, while the major portion separates out and sinks to the bottom of the mold. On cooling and solidification of the steel, the dissolved lead separates out in the form of a highly desirable, uniformly distributed submicroscopic dispersion, but the amount is so small that it has little efiect in increasing the machinability. The major action in this respect comes from the lead which is trapped in mechanical suspension, but this has the serious disadvantage of being present in the form of visible masses and prills of variable size distributed in an irregular and random manner ficiently fine state of subdivision. The practice .tion into the steel.

subsequently forged or rolled, the hot lead thus trapped in the bottom of the ingot will squirt out with great force and may strike and injure the workmen. Another sourceof grave danger to the workmen resides in the excessive amounts of extremely poisonous fumes evolved from the metallic lead upon addition to the molten steel elements, which fusion product is of such a character that the lead is liberated in the molten steel in an atomic or nascent state, productive of an emulsion of lead in the molten steel and of a consequent fine and uniform, submicroscopic dispersion of the lead in the steel on subsequent solidification, thereby avoiding on minimizing the danger of visible globules of metallic lead being trapped in the steel. There also results a higher efficiency of introduction of the lead into the ferrous metal.

According to one modification of. the invention, this lead-containing fusion product comprises an alloy of lead with alkali or alkaline earth metals, of which alloys with sodium, potassium, calcium, barium, and magnesium have proved particularly successful, The use of such alloys is, as stated above, based on the concept of introducing the lead into the molten steel in such form as to facilitate the formation of an emulsion of lead in the molten ,steel, and at the same time to introduce a desired deoxidizing effect therein by virtue of the alkaline metal employed. The disintegration of the lead alloy frees the lead in the atomic state and promotes the formation of the emulsion referred to, whereby upon cooling and solidification of the steel, the lead is uniformly dispersed therein in submicroscopic form. Introduction of the lead into the molten steel in this way avoids the necessity for the violent turbulencerequired where metallic lead is added as such in accordance with prior practice. This is due to the fact that the lead alloy is active chemically, and hence does not require mechanical turbulence for introduc- Also, a much higher efliciency of lead introduction is secured, and with a minimum production of lead fumes. The invention accordingly permits of introducing the lead directly into the ladle.

Instead of introducing the lead in the form of an alloy with alkaline metals, I may, in accordance with a .second modification of the in vention, employ a fusion product of lead, molybdenum and sulfur, the use of which, however, is based on the same general principles as the lead alloy. This product has the added advantage of simultaneously introducing two other elements which promote machinability, namely, molybdenum and sulfur. Furthermore, the presence of these elements has a catalytic effect in furthering the formation of the'lead emulsion in the steel. This fusion product may be made from easily obtainable commercial products, such as lead monoxide (PbO), commonly called litharge, and the naturally occurring sulfide of molybdenum (M082), known as molybdenite. These two compounds, when fused together in appropriate proportions as set forth hereinafter, give a product which is suitably stable for manufacture, and which breaks up satisfactorily when added to the molten ferrous metal, yielding finely divided atomic or nascent lead, molybdenum and'sulfur, which unite with the ferrous metal.

The fusion products aforesaid should preferably be free from excess lead, i. e. lead present in the metallic form as such, since this is productive of visible globules in the steel, That is to say, where the lead is fused with alkaline metals, the proportions should be such as to form a true alloy. Although in the'case of the Pb-Mo-S fusion product, it has not beendefinitely determined whether the lead, molybdenum and sulfur form a true chemical compound, it has been determined that certain proportions result in freedom from excess lead, and tha't,irrespective of this, any excess lead present may be easily separated bycrushing and screening the frit. The frit is thus crushed to a fine powder before adding to the steel.

Alloys of lead and an alkaline metal which are suitable for incorporating lead in a molten bath of ferrous metal in accordance with the invention, will contain about 0.5 to 10.0% of the alkaline metal or metals, by weight of the entire alloy, the balance being substantially lead. The following examples in Table I are illustrative of satisfactory analyses:

Table I Percent by weight of alloy Example Alkaline metal parts, molybdenite.

N t-'9 P!" CHOIOIOOI Er se Appropriate proportions for the starting materials in the preparation ofthe Pb-MoS fusion product are about 11 parts litharge to upwards of about '3 parts, and preferably about 3 to 5 Smaller proportions of molybdenite are productive of excess lead in the product which is undesirable. The product may be made by admixing th'e litharge-and molybdenite and heating, preferably in a crucible, to a suflici'ently high temperature to effect fusion. The fusion temperature increases somewhat with increase in the proportion of molybdenite to litharge employed, as shown in the following Table II, which also gives the PbMo-S analyses of the sintered products obtained, the balance being present as oxygen.

Although, as above stated, the machinability of steel is enhanced by the presence of high sulfur, this is objectionable for other reasons as above referred to. I have found by adding lead to the steel in accordance with my invention, that the sulfur content may be markedly reduced without impairing the machinability, and that, in the alternative, the addition of the lead but without reducing the sulfur, results in a further increase in machinability. The following examples of Table III illustrate these effects as applied to a free machining type of steel ordinarily analyzing about: 0.15% carbon, 1.0% manganese, 0.1 molybdenum, 0.1 sulfur, and the balance substantially iron. In this table, the Machining Index" refers to the machinability of the steel as determined by a carefully standardized drilling test with the indexof 100 representing the machinability of a standard block of annealed carbon tool steel. The machining index as thus determined, of course increases with increase of machinability.

Table III Percelnt lay weig t o Machining Steel index S Pb Comparison of steels 1 and 2 shows that even slight additions of lead while retaining the sul-' fur substantially constant, increases the machinability noticeably. The results for steels 3 and 4 I show that further additions of-lead permit of reducing the sulfur content well before the usual limit of about 0.05% for this. element insteel, without impairing the machinability, In steel 4,

for example, althoughthe sulfur content is exceptionally low, the machinability is nevertheless some 33% better than that of annealed carbon tool steel.

The relative eflicienciesv of :le'ad additions to the steel in accordance with this invention as compared to the prior practice of adding metallic lead, is also evident from the following in" .connection with Table III. For obtaining steel 2, metallic lead in :the form of leadpellets wasadded in amount equal to'0.35% byweight of theresulting steel. Of this amount, only 0.05%-

was retainedin the steel, the balance separating outat the bottom of the ingot,-so that only 7% of the total lead added was retained in the'ste'el.

In steel .,qthc lead was added, in accordance with the present invention, as an alloy of 98.5% lead and 1.5% calcium, lead being thus added as before to the extent of 0.35% .by weight of the resulting steel. Since the finished steel was found on analysis to contain 0.22% lead, the efiiciency of introduction was 63% as compared to but 7% for the aforesaid addition of metallic lead. In steel 4, 0.50% lead was added in the form of the fused PbMo-S product, resulting in a steel containing 0.23% lead, the -efficiency of introduction being thus about 46% in this case.

Table IV below shows the individual and combined effects of both sulfur and lead on the machinability of an SAE 1020 type of steel containing about: 0.2 carbon, about 0.7 manganese, about 0.2 silicon, and the balance substantially iron except for lead and sulfur as indicated.

Table IV Pergcgrtit by Wei 0 Steel lf g 'i S Pb Comparison of steels I, 2 and 3 shows thatthe addition of 0.1% lead has about the same effect in enhancing machinability as an increase of about .03% sulfur; while the result for steel 4 shows the marked further increase of machinabili-ty when the lead is present along with the increased sulfur, still keeping the sulfur, however, at about the upper limit of 0.055% permitted for this type of steel.

In .the production of lead-containing steelsby prove machinability without adding materially to the nonmetallic inclusions in the steel.

I claim:

1. A composition for the introduction of lead in finely dispersed form into ferrous metals and alloys while molten, comprising the fusion prodnot of an oxide of lead and a sulfide of molybdenum.

while molten, a fusion product containing lead 2. A composition for the introduction of lead in finely dispersed form into ferrous metals and alloys while molten, comprising the fusion product of litharge andmolybdenite.

3. A composition for the introduction of lead in finely dispersed form into ferrous metals and alloys while molten, comprising the fusion product of litharge and .molybdenite in the proportions of about eleven parts litharge to upwards of about three parts molybdenite.

4. A composition for the introduction of lead in finely dispersed form into ferrous metals and alloys while molten, comprising the fusion product of litharge and molybdenite in the proportions of about eleven parts litharge to about three to five parts molybdenite.

5. A composition. for the introduction of lead in finely dispersed form into ferrous metals and alloys while molten, comprising a fusion product containing lead, molybdenum and sulfur in the proportions-of about .to lead, about5 to 20% molybdenum and about 3 to 12% sulfur.

6. A method of producing ferrous metals and alloys containing lead in finely dispersed form, which comprises, incorporating in said metal while-molten, the fusion product of an oxideof lead and a. sulfide of molybdenum.

'7. A method of producing ferrous metals and alloys containing lead in finely dispersed form, which comprises, incorporating in said metal while molten, the fusionproduct of litharge and molybdenl te.

8. A method of producing ferrous metals and alloys containing lead in finely dispersed form, which comprises, incorporating in said metal 35 while molten, the product resulting from fusing litharge and molybdenite in proportions of about eleven parts litharge to upwards of about two parts molybdenite. I

9. A method of producing ferrous metals and alloys containing lead in finely dispersed form, which comprises, incorporating in said metal molybdenum and sulfur.

10. A method of producing ferrous metals and 45 alloys containing lead in finely dispersed form, which comprises, incorporating in said metal while molten, a fusion product containing le'ad, molybdenum and sulfur containing about 60 to 30% lead, about 5 to 20 molybdenum and about 50 3 to 12% sulfur.

FREDERICK F. MCINTOSH. 

